| 초록 |
Recently, tremendous efforts have been focused on improving the efficiency of organic light emitting diodes (OLEDs) through either the development of better materials or more efficient device structures. In an OLED, both the holes and electrons are injected from the opposite electrodes then combined together to form excitons. The radiative decay of the singlet excitons is very fast but that of the triplet excitons is usually inhibited by the rule of the spin conservation and is very inefficient. By employing triplet-based phosphorescent dye in small molecule- and polymer-based LEDs where both singlet and triplet excited states participate, the internal efficiency can reach as high as 100%. In the dye doped systems for LEDs, two kinds of emission mechanisms are possible. One is based on the Föster or Dexter type energy transfer from the host to the dopant, the other is based on the direct recombination of the injected charges on the dopant site, i.e. charge trapping mechanism. Therefore, the HOMO and LUMO energy levels of host and dopant are very important to take place both energy transfer mechanisms. Thus, the combination of host and dopant is very important for fabricating efficient device. For the electrophosphorescent device, the triplet energy level of the host and dopant should be carefully considered either. The triplet energy level of the host materials should be higher than that of guest since triplet exciton of phosphorescence guest (e.g. green- and blue- emitting materials) should not be transferred to host material. In this presentation, the importance of the host materials to improve the device efficiency will be discussed. |
